1. Technical Field
The present disclosure relates to calorimeters, and more particularly, to a colorimeter with optical geometry and assembly that provides high a signal-to-noise ratio (SNR) and/or properly handles ambient light.
2. Background Art
Colorimeters are accurate devices for measuring the spectral content of light emitted either directly or indirectly from a given source. Standards bodies such as the Video Electronics Standards Association (VESA) have been a driving force behind the development of colorimeter performance. The VESA 1.0 standard, for example, is primarily directed to specifying measurement of contrast ratio, and is limited to a no greater than +/−2 degree viewing angle from the source to the sensor.
However, typical devices being measured, such as computer displays, are actually viewed from much greater angles than specified in the standards. To further exacerbate this situation, the degradation of spectral parameters that occurs with increased viewing angle, does not occur equally for all colors. Such incompatibilities have yet to be resolved or otherwise addressed. In addition, early devices developed under the standards were relatively costly. Such devices typically employ highly accurate measurement optics. Apertures have frequently been used in conjunction with lenses or other precision optical elements. Typical colorimeter configurations include a light sensing diode, and an integral lens that directs light to the sensor area.
More recent developments have produced designs resulting in low cost calorimeters with performance characteristics approaching or exceeding professional quality required by the standards. Such newer designs generally employ basic aperture geometry, and either non-overlapping or overlapping spectral ranges/filters. One such design uses a unique aperture plate that has oblong holes, and is spaced between the target surface and the filter/sensor set. The resulting field of view is about +−15 degrees or more on each axis.
However, such a design does not correspond to the field of view of the human eye. Rather, it is about four times too large. Also, the position of the aperture plate is spaced from the target surface by a mounting distance that is typically defined by one to four relatively large suction cups (i.e., one at each corner of the colorimeter housing). Such designs generally cause variations in the mounting distance due to leaking-based relaxation of the suction cups.
This variation, in conjunction with a fixed-distance between the aperture plate and the sensor, causes the field of view to vary as well, and adversely impacts the accuracy of color measurements taken.
In addition, colorimeter designs generally require alignment of the sensor to one or more optical paths. This alignment is typically provided by a secondary reference surface on the sensor diode. However, this sensor reference does not assure accurate alignment of the sensor in the plane parallel to the measured surface, and there is no means for locating the center of the senor lens to the center of the optical path. Such limitations contribute to undesirable decay of the SNR, as well as to inadequate color measurement accuracy.
While prior art colorimeters concentrated on measurement of what the sensor “sees”, effects on perception resulting from ambient light impinging upon the source has eluded the existing colorimeter art. Colorimeter performance can be greatly degraded by ambient light effects of: 1) image perception of the human eye in the presence of varying ambient conditions, 2) ambient light reflected from the display screen integrated with the display screen image source, 3) flare, and other factors.
A metaphoric example of the types of perceived changes in “viewed monitor” systems in the presence of changing ambient light conditions follows below. When viewing automotive headlights in the daylight, they appear yellowish and dim. However, when viewing the headlights at night, they appear bright and white or blue white. This example highlights how important it is to calibrate for ambient light conditions in order to get quality results in a monitoring/viewing/measurement system.
In general, viewing a screen in dim ambient conditions leads to a perception of less color. If this in turn leads to arbitrary adjustment of the brightness control (when available such as with analog displays), system calibration can be severely hampered.
An attribute of a good calorimeter is the ability to anticipate the effects of ambient light on the source as perceived by the viewer and correct for such effects. Brightness controls on displays are notoriously non-linear. Uncalled for adjustment of brightness by the user can introduce severe degradation of contrast, linearity and perceived anomalies. Further image contrast and the ability to discern detail at different levels of image luminosity can be adversely affected by ambient illuminance of the area immediately adjacent to the image.
In one instance known to the inventors, a product distributor, LaCie, adapts only for intensity of ambient light, ignoring calorimetric content. However this solution also requires that the sensor be reversed to make the ambient reading. While it has been recognized that ambient light presents many problems and challenges to colorimeter measurement quality, to date the most sophisticated approaches are limited to taking a reading of the ambient at a point, and then depending on that measurement of ambient light for all ensuing ambient light compensation. Typically this is accomplished by turning the calorimeter away from the source image to take a reading of the ambient light. In some instances a diffuser is placed on the colorimeter to improve the specific and single measurement.
Previously disclosed embodiments of a calorimeter by the assignee of the present application measured ambient light by removing the calorimeter body from the image source screen, placing a diffuser on the viewing surface of the colorimeter, and orienting the combination away from the screen to get a reading estimate of the ambient light.
Nonlinearities of displays, complexity of color measurement, response of the human eye, effects of viewing room ambient conditions, measurement degradation due to extraneous other factors and the interactions between the factors make measurement and compensation challenging at the least. It is desired to maximize the S/N (signal/noise ratio) by controlling the ambient light effects within the system such as: flare, reflected light, extraneous light rays impinging on the sensor and numerous other sources of light impinging on the sensor that are not primary source generated rays. Therefore, a colorimeter is desired that takes into account the effects of ambient light impinging on the source image (LCD, CRT screen, etc.).
Further, there is a desire for making the calorimeter measurements agree more with the ambient light that impinges on the source image and reading the ambient light on an ongoing basis to make important corrections in a varying ambient light environment. It is further recognized that it would be a desirable feature of a colorimeter to be able to alert the user to real time changes in ambient viewing light that are deemed sufficient to effect the perceived attributes of the image.
In sum, an improved colorimeter design is desired having optimal or otherwise increased SNR, that has a field of view that corresponds to that of the human eye and that has the ability to continually sense ambient light impinging on the source image. In a more general sense, an improved colorimeter design is desired for accurately measuring color from the likes of displays, CRTs, LCDs, and printed or pictorial materials.